Both utility and industrial users of induction motors have historically been concerned in finding reliable procedures for detecting rotor faults or "cage faults" before such faults become exacerbated to the point of catastrophic motor failure Obviously, such motor failures can force expensive shutdowns of factory production and in certain applications can have adverse safety implications.
Until recently, the detection of rotor faults involved taking the induction motor out of service, disassembly and/or the application of special instrumentation internally and/or externally to the motor For example, U.S. Pat. No. 4,808,932 discloses a broken rotor bar detector which requires the installation of a flux coil wound on one of the stator teeth of an induction motor. Implementation of such a detector requires motor disassembly, unless the flux coil is installed at the time of manufacture or rewinding. U.S. Pat. No. 4,761,703 discloses a rotor fault detector based on motor current analysis which requires the stationing of a flux coil at an appropriate external position proximate the motor to pick up stray axial flux in order to precisely determine motor speed. This approach does not require motor disassembly, or, in many cases, an interruption in service. However, in some motor applications it is not convenient from the standpoint of accessibility or there is no space available to station the flux coil in an appropriate external flux detection position.
It is accordingly an object of the present invention to provide a method and apparatus for detecting rotor faults in induction motor which is totally non-invasive of the motor operating environment.
A further object is to provide a rotor fault detection method and apparatus of the above-character which does not require motor disassembly or even service interruption.
An additional object is to provide a rotor fault detection method and apparatus of the above-character, which relies solely on an analysis of motor current which can be sensed at a point removed from the motor operating environment, such as at its motor control center
Another object is to provide a rotor fault detection method and apparatus of the above-character, which avoids false indications of rotor faults
Yet another object is to provide a rotor fault detection method and apparatus of the above-character, which is capable of reliably providing an early indication of rotor faults, thus enabling the user to schedule convenient service and repair well in advance of any catastrophic failure.
Other objects of the invention will in part be obvious and in part appear hereinafter.